As is well known, the many operations that have to be carried out on such assembly lines, e.g. on the cabins of aircraft when fitting the panelling with which they are to be covered or on certain parts of their engines, require the use of a very large number of different screws especially designed for each use and each having to be fitted in a precise location, these screws having to meet close tolerances and satisfy stringent quality criteria.
As a result of this, these screws have quite a high cost price, which makes it particularly desirable that losses of screws in the course of these operations be as small as possible.
In addition, owing to the large number of references employed with these screws, it is vital that ceaseless rigorous checks be maintained by the technicians working on the line in order to ensure that a screw that is not exactly appropriate is not used in the place of another screw which does not quite have the characteristics required.
In the usual method used in this kind of work, these conditions are not as a rule very adequately met. In practice, the screws are presented loose in boxes or similar containers which are placed close to each other on a bench or appropriate support of the same kind, set at an appropriate height near the cabin or engine on which the operations are to be carried out, in order that the fitters can pick the screws up as ergonomically as possible, though having to select from the different boxes in order to take from one of them the only correct screw for the intended purpose.
For this reason, owing to the adjacent arrangement of the various boxes, mixing is possible, if not frequent, as the screws, which are usually short and of small diameter, do not individually include a code etched or otherwise applied to each of them by which they could be identified. Besides, the size of the screws would mean that any such code would have to be in tiny characters and/or figures, making them difficult if not virtually impossible for the user to read, quite apart from the loss of time that this would incur.
Lastly, with the conventional system of adjacent boxes containing loose screws which may be very similar but are not identical and may therefore give rise to confusion which could have serious consequences, especially if an unsuitable screw is used instead of another, the technician may also relatively frequently drop such a screw, for example while taking another screw from a particular box, or by clumsiness. These screws fall to the ground from the workbench and are never gathered up for reuse because of the importance of maintaining strict cleanliness and because of even the small risk of deformation which the impact of the fall may have occasioned.